Microcapsules are employed to a large extent in the perfumery and flavoring industries. They constitute delivery systems for perfuming or flavoring ingredients and can be advantageously used in a very large number of applications. The encapsulation of active substances such as perfuming or flavoring ingredients provides at the same time a protection of the ingredients there-encapsulated against “aggressions” such as oxidation or moisture and allows, on the other hand, a certain control of the kinetics of flavor or fragrance release to induce sensory effects through sequential release.
Now, the numerous advantageous properties of microcapsules in these fields are opposed to other properties that must be taken into account during their preparation, transportation, storage and handling. In fact, such delivery systems, due to their nature, and in particular to the fact that they encapsulate volatile and flammable substances, constitute combustible dusts which can, when dispersed in air or another oxygen-containing gas, form readily ignitable mixtures. When ignited by a sufficient powerful ignition source, the result is a rapid combustion reaction with advancing pressure and flame front.
This issue becomes important during the preparation of microcapsules. In particular, spray-drying and fluidized-bed encapsulation processes are highly concerned by this issue, as they are both based on the use of an equipment wherein particles are suspended in hot air as fine particles and can therefore undergo explosion during their preparation.
Spray-drying is the most common encapsulation technique used to stabilize volatile substances such as flavors or fragrances, by encapsulating them in a solid form that is suited to many applications. Spray-dried powders are commonly made in the usual spray-drying equipment. Spray-drying is usually effected by means of a rotating disc or of multi-component nozzles. Detailed techniques are described for instance in K. Masters, Spray-drying Handbook, Longman Scientific and Technical, 1991.
Fluidized beds are used for spraying a coating on a core material fluidized in a bed. This encapsulation technique is also well known and is described for instance in European patent application 70719 or in U.S. Pat. No. 6,056,949, the contents of the latter of which is hereby expressly included herein by reference to the extent necessary to understand this technique.
Both above-described encapsulation equipments are susceptible to explosions of particles suspended in the air, so that they thus have to be adapted as a function of the technical safety parameters characterizing the particles there-treated. In particular, they have to be dimensioned as a function of the violence of explosions that can occur during the preparation of microcapsules. Therefore, the problem of reducing the violence of possible explosions of powder products resulting from such encapsulation processes is of paramount importance for the industry.
For the safe handling of combustible substances, it is imperative to know the dangerous properties of a product. One reliable way to characterize the combustible and explosive properties of a product is to subject a sample of the product to various tests and classify the results in accordance with the technical safety characteristics. The international standards (VDI Guideline 2263 part 1: Dust Fires and Dust Explosions, Hazard Assessment—Protective Measures, Test Methods for the Determination of Safety Characteristics of Dusts, Beuth, Berlin, May 1990) describe the test equipments (Modified Hartmann apparatus and Close apparatus) and methods. These methods allow to determine physical constants such as the maximum explosion behavior of a combustible dust in a closed system. A pyrotechnic igniter with a total energy of 10 kJ is used as ignition source. From test methods described in the mentioned guidelines, a characteristic constant, K−St, which is dust specific, is determined. As there are so many such dusts produced and processed in industrial practice, for example for pharmaceutical and cereal or flour products, it is appropriate to assign this maximum explosion constant to one of the several dust explosion classes and to use these as a basis for the dimensioning of constructional protective measures. The correspondence between these classes hereafter referred as dust hazard classes, and the constant K−St is the following:
Dust Hazard ClassProduct Specific Constant K-St [bar · m · s−1]St-1>0 to 200St-2<200 to 300St-3<300
Now, despite that some perfuming and flavoring ingredients are classified in a dust hazard class St-1, a large number of these ingredients and thus the microcapsules encapsulating them, and depending on the volatility of the perfuming or flavoring ingredients, are still classified under an St-2 dust hazard class and thus require production equipment specifically adapted to contain or withstand the violence of possible explosions, which of course can be very costly.
While solutions have been proposed for solving similar problems in other technical fields, such as for instance for polymeric organic compositions which demonstrate a tendency to degrade, the perfuming and flavoring industry was never provided with an efficient solution, adapted to these products and which would solve the economic problem related to the costly equipment required to prepare St-2 classified microcapsules. The present invention now provides a solution to this problem.